1. Field of the Invention
The invention relates to an apparatus for drying material and more particularly relates to an apparatus and method for preventing the build-up of undesirable materials in the apparatus under relatively high humidity drying conditions.
2. Brief Description of the Prior Art
In the drying of material in a flowing gas stream in a continuous operation, if the gas stream includes solids or highly viscous materials therein, there is a tendency for these materials to deposit and build up in areas of the equipment where velocity is substantially reduced. This settling or build-up generally occurs where relatively sharp turns are experienced in the gas flow through the drying equipment.
For example, in the drying of tobacco, tobacco generally enters drying equipment relatively high in moisture and includes flavorings and the like, commonly referred to in the tobacco industry as "casings," in the gas stream which is processed in the drying equipment. During drying, build-up occurs in the equipment, particularly at points where the particle velocity is substantially reduced.
In the drying of cut tobacco, the initial moisture content usually ranges from about 15 to 35 percent for the lamina, and from about 20 to 60 percent for the stem. This tobacco is dried in the presence, generally, of hot air until the moisture is in the range of from about 12 to 15 percent. Several devices are known in the prior art and two examples of drying cut tobacco are disclosed in U.S. Pat. No. 3,357,436 and U.S. Pat. No. 4,167,191.
In recent years, it has become a widespread practice in the tobacco industry to expand or "puff" tobacco prior to incorporation into a cigarette product. This expansion or "puffing" leads to better economics as well as a lowering of the tar and nicotine in the final product. Many different techniques are described and known in the prior art for expanding tobacco, such as impregnation of the tobacco with water, an organic liquid, carbon dioxide, or ammonia, and then subjecting the impregnated tobacco to temperatures or pressures sufficient to then liberate the impregnant from the tobacco. However, in many of the drying techniques utilized for expanding the tobacco, many of the advantages attributed to the expansion technique are lost because of the shrinking during the drying process. Furthermore, in the drying of the tobacco at relatively high humidity there is a tendency for the viscous casings to settle out and plug up the drying equipment. Thus, an apparatus which eliminates the problem or at least reduces the problem of build-up is of substantial benefit in the processing of expanded tobacco particles.